This invention relates to atomic clocks and in particular to cesium beam frequency standards and to means for eliminating microwave power fluctuation induced frequency errors in such devices.
An atomic clock is an electronic clock whose frequency is supplied or governed by the natural resonance frequencies of atoms or molecules of suitable substances. It is the most precise of all clocks. U.S. Pat. No. 3,076,942 entitled Molecular Beam Frequency Standard issued to J. H. Holloway et al, Feb. 5, 1963 as well as other publications describe these devices in detail. They are commonly used in navigation systems such as Loran C; deep space communications and Doppler navigation; and collision avoidance systems for aircraft. All of these and other applications require extremely precise time measurements. It has been observed, however, that frequency drift of the order of several parts in 10.sup.13 per year commonly occur in some atomic clocks such as the commercial cesium beam frequency standard. Accordingly, there is an important need to obtain clocks with better long term stability than is now available. Such a need is evidenced by the Department of Defense's Global Positioning System, a multi-billion dollar navigation system to be in operation in the next decade.
Dispersion between various national time scales, such as USNO and NBS is also significantly affected by long-term instabilities in cesium beam frequency standards. A significant cost savings can be achieved as a result of the present invention due to a much less frequent need to carry portable clocks as it is now needed for synchronization of Loran C and Omega navigation systems, and of various satellite tracking stations. Heretofore, a minimum of two portable atomic clock trips has been necessary to transfer frequency with the necessary accuracy. It is anticipated that improvements in atomic clocks provided by the present invention will provide accurate frequency transfer from only one trip; a very important consideration for remote tracking sites. Such improvements reduce the environmental sensitivity of atomic beam frequency standards thereby improving significantly the stability of a portable clock subject to environmental fluctuations far in excess of ideal laboratory conditions.
The present invention is also directed toward allowing a reduction in the degree of environmental stability required for laboratory clock ensembles, such as NBS and USNO, for example.